The invention relates to a method for determining wear and more particularly a method of using color as an indicator of the degree of both barrel and screw wear in plastic processing equipment such as extruders and reciprocating injection molding machines.
Extruders and injection molding machines use a screw to feed material through the machines. The screw has a flight and a channel. In addition, the screw is located inside a barrel. As the screw rotates inside the barrel, the flight advances the material through the barrel. For plastic material, the screw flight is designed to cause the material to melt as it advances through a heated barrel. In addition, the screw may be used to mix two or more materials as they are moved through the barrel. Furthermore, the machine may use several screws.
The machine is designed and manufactured with a predetermined distance between the screw flight and the barrel. During use, the movement of material through the screw and barrel causes wear which increases the distance between the screw flight and the barrel. If the material is abrasive, the wear occurs more quickly.
Generally an increase in wear creates a reduction of the shear rate on the polymer, resulting in less mixing to occur in the barrel. This results in molten polymer with poor homogeneity. A non-homogeneous product causes the risk of fabricating plastic parts which will fail to meet performance criteria established on the physical properties. Preventive maintenance procedures to monitor screw and barrel wear presently used by the industry require machines to be shut down, disassembled, and physically measured to determine actual wear. This process is time consuming, and generally requires one-half to one full day of lost productivity and the use of high cost maintenance labor, causing the process to be expensive.
A method is needed which can be more efficient and more economical for the determination of screw wear.
In general, the present invention provides a facile method for evaluating the extent of wear, and for determining when to repair, plastic processing equipment including a screw and barrel, by first establishing a color standard and then evaluating what color results when color samples (made from two differently colored thermal plastics) are processed in such equipment at spaced intervals over time. Because of the decreasing homogeneity that results as the mixing in such equipment deteriorates over time through wear, the resulting color will correspondingly deteriorate in comparison to the color standard and in relation to color samples obtained using equipment at earlier times. The color of the samples can be subjectively and/or objectively evaluated to provide a basis for determining when the equipment should be repaired.
Pursuant to one illustrative embodiment, a first material of a first color and a second material of a second color are fed into the machine to create a resultant third material with a third color. The resultant third material is either extruded or is injected into a mold to form a part.
More specifically, in this embodiment, a precolored thermoplastic compound is fed into a machine with a custom formulated concentrate (or master batch) of a different color. When these two products are mixed homogeneously, these products will produce a fabricated plastic part uniformly colored in an entirely different shade than the colors of either component.
The color of the part is then measured with a measuring device, such as, for example, a spectrophotometer. The color will change as the screw and barrel wear. The color value obtained may then be compared with previous color values, obtained at other times during the equipment use, to determine the amount of wear.
Specifically, the method would be performed when the machine is new (or first used or repaired) in order to obtain an initial or first color value. The first value would be placed on a graph with operating time along the X axis and color along the Y axis. After the specified interval of operating time, the method would be performed again to obtain a second color value and this second value would be placed on the graph. Similarly, the method would be performed after each specified interval of operating time to obtain the third value, fourth value, etc. The values would then be connected with a line to form a curve. The user would analyze the curve to determine when the color and wear has diminished below an acceptable level. The machine would then be disassembled and rebuilt.
In this fashion, a straightforward method for evaluating plastic processing equipment wear is provided which avoids the expense and down time of prior techniques. This method can be carried out in a number of different ways, providing as rigorous and objective analysis as the user considers appropriate for the particular equipment repair sequence. Moreover, this method can take advantage of color compounders who have the ability to insure that the proper color samples are used each time for this comparative method, as well as to provide the color change analysis and determination, as the equipment user deems appropriate.